A direct-conversion IQ modulator using a 25% duty cycle is known in the art.
One example of a transmitter of this kind consists of a baseband signal generator, baseband filter, passive mixer and a sub-circuit. The sub-circuit may be a further mixer, an amplifier (or driving amplifier DA), or other circuit connected before an antenna. In the following description, an amplifier is assumed for the sub-circuit. The input gates of the amplifier may be high impedance inputs with some parasitic capacitance. Any capacitance will cause charge to be stored on the high impedance amplifier input gate in each time period (or phase) of the operation of the passive mixer, and this charge may be fed back onto the filter output via the mixer's switching mechanism, in the subsequent time period. Due to the order of the mixer periods/phases (I(+), Q(+), I(−), Q(−)) the fed-back charge causes undesired cross-coupling between the I-channel and the Q-channel. In the example of FIG. 1, the cross-coupling occurs at the output of the baseband filter, which has the effect that the frequency response of this filter may be shifted by some finite offset frequency.
With respect to FIG. 1, part of an example known transmitter structure is shown schematically. The transmitter comprises a baseband filter 11. The inputs to the baseband filter may come from a Digital to Analogue Converter (DAC) (not shown). The baseband filter 11 as shown in FIG. 1 comprises an in-phase (I) filter 13 with positive I(+) and negative I(−) differential outputs. Furthermore the baseband filter 11 comprises a quadrature-phase (Q) filter 15 with positive Q(+) and negative Q(−) differential outputs. Herein, “positive” and “negative” inputs and outputs refer to non-inverting and inverting differential inputs and outputs, respectively. As will be apparent to those skilled in the art, the labelling of one terminal in a differential circuit as “positive” and the other terminal as “negative” is essentially arbitrary.
The transmitter further comprises a passive mixer 21. The baseband filter outputs provide the inputs to the passive mixer. The mixer 21 comprises pairs of switches configured to switch or selectively couple the positive and negative I differential inputs I(+) and I(−) and positive and negative Q differential inputs Q(+) and Q(−) to the mixer differential outputs. The mixer operates in such a manner that the output of the mixer for a full mixer cycle can be divided into four time periods or phases. These phases are determined by the local oscillator signals shown in FIG. 2, which control the switches. The first phase occurs as switches 22, 25 (controlled by local oscillator signal LO1) couple the positive and negative differential in-phase inputs I(+) and I(−) to the mixer first and second differential outputs, respectively. The second phase occurs as switches 26, 29 (controlled by local oscillator signal LO2) couple the positive and negative differential quadrature phase inputs Q(+) and Q(−) to the mixer first and second differential outputs, respectively. The third phase occurs as switches 23, 24 (controlled by local oscillator signal LO3) couple the positive and negative differential in-phase inputs I(+) and I(−) to the mixer second and first differential outputs, respectively. The fourth phase occurs as switches 27, 28 (controlled by a local oscillator signal LO4) couple the positive and negative differential quadrature phase inputs Q(+) and Q(−) to the mixer second and first differential outputs respectively. Having completed one cycle of first, second, third and fourth phases, the mixer may then repeat the cycle. The mixer as described herein outputs the switched signals at the mixing (LO) frequency to the sub-circuit 31. In the process, it also combines the in-phase and quadrature-phase differential signals into a single pair of differential outputs, which are coupled to the inputs of the sub-circuit 31.
The transmitter further comprises a sub-circuit 31, for example an amplifier (or driving amplifier DA) configured to receive and combine the mixer 21 differential outputs. The sub-circuit 31 is shown in FIG. 1 as a differential amplifier with the gates of the amplifier's input transistors 35 and 37 coupled to the mixer and configured to receive the mixer 21 outputs. The remainder of the amplifier circuit (including the antenna load) is indicated schematically by block 36, coupled to the transistors 35 and 37. The parasitic capacitances CIN(+) and CIN(−) are indicated in FIG. 1 by the capacitors 33 and 39, coupling the gates of transistors 35 and 37, respectively, to ground.
The input capacitance of the sub-circuit 31 is charged during each LO phase and this charge is fed back to the mixer input via the switching mechanism of the mixer. This can be demonstrated, for example, by the capacitors 33, 39 at the end of the positive in-phase I(+) part being charged to voltages VI(+) and VI(−) respectively. As the mixer switches between the first period and the second period, the charge associated with voltages VI(+) and VI(−) from the first period is then fed back to the mixer quadrature-phase inputs Q(+) and Q(−).
This feedback of charge, between periods or phases of the mixer cycle, may cause asymmetry in the operation of the transmitter. Not only is there crosstalk between the I-channel and Q-channel, but the feedback charge polarity is different in different phases—the charge being “added” after some of the transitions (namely, at the start of phases LO2 and LO4), but being “subtracted” after the other transitions (namely, at the start of phases LO1 and LO3).
It is known to minimise this effect by increasing the amplifier input capacitance relative to the mixer input capacitance, but increasing the capacitance does not solve the problem and furthermore is problematic in that it slows down the operation of the amplifier and therefore degrades the performance of the transmitter.